Last month: We made a series of modifications, testing for mileage gains and/or horsepower differences. Our tests included upgrading to synthetic fluids, adding a turbo back exhaust system, adding an air intake system, use of a programmer with performance chip, and cleaning up the air flow with a hard tonneau cover. We found all of the listed products did improve mileage to some degree, and some added performance as well. Our stock F-250 got 17.2 mpg when we started, making 250 hp at the rear wheels. After we were done, we got 23.92 mpg, with 411 hp, an improvement of 161 hp.
This month, we'll look at the costs of the modifications, and give you an idea how you could make these modifications on your own.

The prices below are the manufacturers suggested retail prices. Many of these items can be used for more than just the 12,000 miles the calculations below are based on, and provide additional positive benefits for your truck beyond fuel economy.

Test #1: Royal Purple Synthetic Fluids
Retail cost of 16 quarts (one oil change for the F250) of 15w40 $ 118.72
Retail cost of 6 quarts (2.5 for front and 3 for back) of Max Gear $ 67.38
Retail cost 2 quarts of Max ATF $ 19.28
Total cost of fluids upgrade $ 205.38
Average number of oil changes needed for 12,000 miles (mineral oil)  2.5
Average cost of an oil change for a diesel truck $80.00
Total average cost of oil changes over 12,000 miles $200.00
Number of oil changes needed with Royal Purple 1
Average cost of an oil change for a diesel truck (with filter) $130.00
Total cost of oil changes with Royal Purple for 12,000 miles $130.00
Cost savings of Royal Purple vs. Mineral Oil $70.00 
Total average cost (using $2.69 per gallon) of diesel fuel for 12,000 $1,879.75
Total average cost (using $2.69 per gallon) of diesel with RP for 12,000 $ 1,706.07
Total savings with Royal Purple (both fuel & oil changes) $ 243.68

Test #2: Magnaflow Exhaust System
Retail cost of exhaust system $ 886.00
Total average cost (using $2.69 per gallon) of diesel fuel for 12,000 $ 1,879.75
Total average cost (using $2.69 per gallon) of diesel with Magnaflow $ 1,794.23
Total savings with increased fuel economy with exhaust system only $ 85.52

Test #3: Airaid Filter
Retail cost of intake kit  $ 398.28
Total average cost (using $2.69 per gallon) of diesel fuel for 12,000 $ 1,879.75
Total average cost (using $2.69 per gallon) of diesel with Airaid $ 1,794.23
Total savings with increased fuel economy with intake system only $ 85.52

Test #4: Quadzilla
Performance Technologies
Retail cost of performance chip $ 799.00
Total average cost (using $2.69 per gallon) of diesel fuel for 12,000 $ 1,879.75
Total average cost (using $2.69 per gallon) of diesel with Quadzilla $ 1,560.20
Total savings in fuel economy with performance chip only $ 319.55

Test #5: Truck Covers USA
hard tonneau cover
Retail cost of tonneau cover $1,695.00
Total average cost (using $2.69 per gallon) of
    diesel fuel for 12,000 $1,879.75
Total average cost (using $2.69 per gallon) of diesel with
    cover for 12,000 $1,716.22
Total fuel savings with tonneau cover only every 12,000 miles $163.53

Overall Costs
Retail costs of all the products together $3,778.42
Total average cost (using $2.69 per gallon) of
    diesel fuel for 12,000 $1,879.75
Total average cost (using $2.69 per gallon) of diesel
    with products for 12,000 $1,326.17
Total savings with increased fuel economy every 12,000 miles $553.58